Surprises from the top of the mantle transition zone

Abstract

Recent studies of chromite deposits from the mantle section of ophiolites have revealed a most unusual collection of minerals present as inclusions within the chromite. The initial discoveries were of diamonds from the Luobosa ophiolite in Tibet. Further work has shown that mantle chromitites from ophiolites in Tibet, the Russian Urals and Oman contain a range of crustal minerals including zircon, and a suite of highly reducing minerals including carbides, nitrides and metal alloys. Some of the minerals found represent very high pressure phases indicating that their likely minimum depth is close to the top of the mantle transition zone. These new results suggest that crustal materials may be subducted to mantle transition zone depths and subsequently exhumed during the initiation of new subduction zones—the most likely environment for the formation of their host ophiolites. The presence of highly reducing phases indicates that at mantle transition zone depths the Earth's mantle is ‘super’‐reducing.

The recycling of chromitites in ophiolites from southwestern North America

“…These are then carried though the upper mantle on the upwelling plume to be incorporated in the magma system beneath a spreading system. However, as stressed by Rollinson (2016), this model requires the…”

Coexisting High-Al and High-Cr Chromitites in the Dingqing Ophiolite (SE Tibet): Inferences to Compositional Heterogeneity in the Tethyan Upper Mantle

“…Identification of ultra-high-pressure minerals such as diamond and moissanite within chromitite, coupled with the recognition of ancient recycled continental lithospheric mantle components, demonstrates the potential contribution of deep mantle processes, including mantle plume activity, to the formation of podiform chromite deposits [14][15][16][17][18][19][20][21]. These findings suggest that the origin of podiform chromite deposits may involve complex interactions between shallow oceanic lithospheric mantle and deeper mantle processes.…”

Ophiolites, diamonds, and ultrahigh-pressure minerals: New discoveries and concepts on upper mantle petrogenesis

“…Chromites containing inclusions of microdiamond, moissinite, other UHP minerals, and crustal material (e.g., zircon, tourmaline) and their host peridotites are transported from the MTZ depths to shallow mantle levels beneath oceanic spreading centers by asthenospheric upwelling (Fig. 1), associated either with slab rollback-induced return-channel flow in the upper plate of a newly initiated subduction zone (Husson et al, 2009;Alfonso and Zlotnik, 2011;Rollinson, 2016) or with a superplume event (Brunet and Yuen, 2000;Maruyama et al, 2007). As an old and dense oceanic lithosphere plunges into the mantle during the establishment of a new subduction zone, gravitational pull of this descending slab causes it to roll back, a process that triggers asthenospheric mantle flow and upper plate extension beneath a forearc setting (Brun and Faccenna, 2008;Schellart, 2008;Husson et al, 2009).…”